1. Field of the Invention
This invention relates to a drive system for motor vehicles having driven front and rear axles, comprising a liquid friction coupling, which is incorporated in the rear axle drive train, and a clutch, particularly an overrunning clutch, which is connected in series with said liquid friction coupling and arranged to be released in response to a front axle speed exceeding the rear axle speed.
2. Description of the Prior Art
Such a drive system is already known from Published German Application No. 33 17 247. The clutch consisting particularly of an overrunning clutch serves to automatically interrupt the drive train during a braking of the vehicle so that a decrease of the surface speed of the front wheels resulting from the braking will not be transmitted by the drive train to the rear wheels. To ensure a drive of the rear wheels also during a reverse travel, the clutch preferably consisting of an overrunning clutch must be bridged. For that purpose a separate mechanism was previously provided, which was actuated whenever the reverse gear was selected.
It is known that constraining torques between the front and rear axles will occur in vehicles having a liquid friction coupling in the drive train when such vehicles are cornering through very narrow bends and that such constraining torques will have the result that the coasting distance is reduced and that the required steering force may be increased. The requirement for stronger steering forces is particularly inconvenient to the driver when he must corner a narrow bend while reversing, as is often the case. For this reason the known arrangement in which the overrunning clutch is automatically bridged during any reversing, as proposed in Published German Application No. 33 17 247, is not desirable.
It is also known in practice to provide liquid friction couplings which are so designed that under certain operating conditions the torque will be transmitted not only by the shearing forces in the viscous liquid but also directly by mixed friction between the disks of the coupling. That mode of operation is required in order to avoid an overtemperature in the coupling. But in order to minimize the wear of the disks under that operating condition, disks having an adequate size must be provided in an adequate number and must have a special shape and surface coating so that their manufacturing costs will be much higher than those of the conventional disks made simply by stamping. Moreover, there will be a higher tire slip of the front wheels of the motor vehicle until the liquid friction coupling assumes the operating condition involving mixed friction and said increased tire slip will adversely affect the cornering stability of the vehicle.